Chromium coatings on components of turbine engines are used to protect component surfaces from oxidation, erosion, and corrosion attack along the hot gas path during gas turbine operation. Conventional chromium diffusion layer formation methods involve the use of chromium pack cementation processes, which are described, for example, in Leferink et al., Chromium Diffusion Coatings on Low-Alloyed Steels for Corrosion Protection Under Sulphidizing Conditions, VGB Kraftwerkstechnik, 73:3, 1-14 (1993). On an industrial scale, the pack cementation process is carried out by placing entire objects to be coated in a pack, which is a fine powder mixture. The pack consists of an inert filler, which is usually Al2O3, an activator, which is usually NH4Cl or other halides, and chromium powder. A metal powder other than chromium powder may be used if the desired diffusion coating is not a chromium diffusion coating. For example, aluminum powder may be used to form an aluminum diffusion coating or silicon powder may be used to form a silicon diffusion coating. The pack, together with the object to be coated, is then heated, usually in an inert or reducing atmosphere. This heat treatment step is usually performed at about 800 to about 1100° C. for about 4-24 hours and it creates a diffusion coating of about 10-80 μm thickness on a suitable metal-based surface.
During heating of the pack, the activator is cracked according to the following example reaction, wherein the metal powder is a chromium powder and the activator is NH4Cl and wherein “s” stands for “solid” and “g” stands for “gas”:NH4Cl(s)→NH4Cl(g)→NH3(g)+HCl(g)  (1)2NH3(g)→N2(g)+3H2(g)  (2)The hydrochloric acid that is formed then reacts with chromium, mainly in the following reaction:2HCl(g)+Cr(s)→CrCl2(g)+H2(g)  (3)
Additionally, a small amount of CrCl3 is formed. The partial pressures of the chromium chlorides (i.e., CrCl2 and CrCl3, jointly referred to as CrClx) are high enough to partially transfer via a gas phase to the metal-based surface when heated at approximately 1100° C. At the metal-based surface, the CrClx are converted in the reducing environment into metallic Cr and gaseous hydrochloric acid, as shown below:CrClx(g)+½xH2(g)→Cr(s)+xHCl(g)  (4)
The gaseous hydrochloric acid then comes into contact with the Cr powder, where it reacts again and is converted to CrClx as shown in reaction 3 above. Thus, the activator is a necessary ingredient in a diffusion coating preparation process. The use of the activator leads to the formation of substantial amounts of gaseous hydrogen halides in the conventional pack cementation process.
There are numerous disadvantages to the conventional pack cementation process. The use of a large amount of activator results in a formation of large amounts of gaseous hydrogen halides, which are intermediate products of the process as discussed above. These hydrogen halides can attack and damage surface coating layers, such as PtAl layer, during coating or repair processes. The hydrogen halides, together with metal halides gaseous phases may also cause the formation of diffusion coatings on surfaces where one would not want to form diffusion coating. Moreover, the generation of hydrogen halide gases can cause environmental health and safety issues.
There are also other disadvantages to the conventional pack cementation process. There is an elevated cost due to a need to use large amounts of a metal powder, an activator, and a filler to create the pack. The pack cementation process also requires use of masking tools and materials, which further add to the cost. Another factor that increases the cost of the pack cementation process is that its heat treatment step requires use of very high temperatures, typically at around 1100° C. Accordingly, an improvement over the conventional pack cementation process is desired.